The present exemplary embodiment relates to color measurement. It finds particular application in conjunction with image rendering devices wherein colors are rendered with a higher number of colorants or color dimensions than are included in an available color measurement signal, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Most readily available color sensors are based on the three primary colors (i.e.; red, green and blue). Light reflected from an image or object is filtered or refracted according to its wavelength. Light sensors such as photo diodes and photo transistors or charge coupled devices are positioned to receive the filtered or refracted light. The sensors generate signals according to the intensity of the light that reaches them. Some sensors report these color signals in terms of red, green and blue intensities. Other sensors include processing elements for converting the received intensities into values in a device independent color space, such as that known as L*, a*, b*. In either case, these color sensors report color measurements in three dimensions (e.g., R, G, B or L*, a*, b*).
Most color printers or rendering devices produce images using at least four colorants. For example, many color printers and color copiers render images using cyan (C), magenta (M), yellow (Y) and black (K) toners or inks to render images. Therefore, for purposes of process control, machine diagnostics and machine monitoring, it would be beneficial if color error measurements were available in terms of the four dimensions or separations (CMYK) in which these rendering devices operate. However, color sensors that report color in four color space dimensions (e.g., CMYK) are not readily available and converting three dimensional color space color measurements to four dimensions is problematic. The set of equations for converting a three dimensional color description to a four or more dimensional color description is underdetermined. Any particular measurement or error measurement in a three dimensional color space can correspond to an infinite number of combinations of colorant in a four or more dimensional rendering device color space. For example, from a purely mathematical standpoint an error measured in RGB or L*, a*, b* can be associated with, for example, a combination of errors in the amount of cyan, magenta and yellow colorant laid down in an image or may be associated with an error in only the black separation or dimension.
Due to this uncertainty, there has been no known (general) method to convert from a difference between the measured (scanned) trichromatic values and desired trichromatic values to a difference between the specified or input colorant values (typically CMYK) and the amount of CMYK that actually or apparently adhered to a target sheet or page. The only available alternative method for determining a difference between desired colorant amounts and actual colorant amounts, in systems that use four or more colorants, has been to introduce at least one additional sensor. However, this adds to system design, manufacturing and servicing costs.
Therefore, there has been a desire for a method for calculating error in an N-dimensional color space (e.g., the four dimensions of a CMYK rendering device color space) from errors determined in an N−1 or fewer dimensional color space of a color sensor (such as, for example, a three dimensional RGB or L*, a*, b* color space reflectance sensor).